Preheat oven to 350 degrees and line 2 baking sheets with parchment paper.
In a medium bowl, cream butter and brown sugar together until smooth and light colored.
Add sea salt and baking soda and stir.
Add egg and vanilla extract and mix until just combined.
Stir in cocoa powder.
Add flour, half at a time and mix until just combined.
Using a 3 TBS cookie scoop, scoop dough and place 2 inches apart on the cookie sheet.
Bake for 12 minutes, rotating halfway through.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.
Store cookies in an airtight container for 4-5 days or freeze.